Earth Moved - Why No Big Tsunami?

The magnitude 8.6 earthquake that struck North Sumatra, Indonesia, at 2:38 p.m. local time today did not lead to a tsunami comparable to Indonesia's 2004 disaster for a couple of reasons.

The magnitude 9.1 earthquake that struck in 2004 and triggered a tsunami that killed more than 200,000 people in 14 countries was a subduction megathrust earthquake closer to shore (250 kilometers south-southeast of Banda Aceh). A megathrust earthquake is also the same type of earthquake that struck off Japan in 2011. These types of earthquakes do the kind of damage that their name implies, mega thrusting of the fault.

Today's temblor ripped along an oceanic transform fault, tearing the seafloor in a strike-slip motion as opposed to popping it apart. Strike-slip motion carries less of an ability to push up on the entire water column above it and hence is less likely to form the same kind of tsunami wave as what hit Indonesia in 2004 and Japan in 2011, though it is not without risks. Early calculations showed that the island of Simeulue might expect waves as high as 6 meters (20 feet).

Coupled with the method of the quake, the power of today's 8.6 magnitude quake was not nearly as strong. The 2004 quake was a magnitude 9.1, making it the third-most powerful earthquake ever recorded.

A tsunami watch and warning was put into effect for the Indian Ocean, immediately after the event. The power went out in Banda Aceh, where residents moved to higher ground, reported Sutopo Purwo Nugroho, a spokesman for the Indonesian National Disaster Management Agency. The island of Simeulue, Prih Harjadi and other coastal areas of Aceh were put on alert.

India also took caution on Andaman and Nicobar Islands, where authorities ordered people to move out of low-lying areas, CNN reported.

As it turned out, sea level gauges show that several islands in the region experienced a rise in sea level from tsunami waves of less than 0.2 meters (1 foot). Indeed, the highest waves recorded struck the coast of Meulaboh 12 minutes after today's earthquake, with a run-up of 1.06 meters (3.5 feet).

No additional tsunami risk followed an aftershock of magnitude 8.2 that struck at 16.4 kilometers (10 miles) deep, 617 kilometers (383 miles) south-southwest of the coast of Banda Aceh.